Pharmacodynamics of piperacillin/tazobactam against Pseudomonas aeruginosa: antibacterial effect and risk of emergence of resistance

Piperacillin/tazobactam has a broad spectrum of antibacterial activity, which makes it suitable therapy for severe sepsis in the hospital setting, especially in critical care. 1 Despite this, there remains a paucity of published preclinical pharmacodynamic (PD) characteristics of piperacillin/tazobactam to aid our optimization of dosing. 2 Studies indicate that the pharmacokinetic/PD (PK/PD) index of effectiveness for β -lactams is the percentage of time that the free drug concentration remains greater than the MIC, fT > MIC . 3 A PD index target of fT > MIC ≥ 50% is widely used to define antibacterial breakpoints for piperacillin/tazobactam, but the appropriate PD index targets for critical care patients is less clear. 4 In critical care, alternative β -lactam targets have been employed, such as exceeding the β -lactam trough concentration by 4 × the pathogen MIC. 5 PD factors have also led to the adoption of prolonged or continuous infusion therapies of piperacillin

Piperacillin/tazobactam has a broad spectrum of antibacterial activity, which makes it suitable therapy for severe sepsis in the hospital setting, especially in critical care. 1 Despite this, there remains a paucity of published preclinical pharmacodynamic (PD) characteristics of piperacillin/tazobactam to aid our optimization of dosing. 2 Studies indicate that the pharmacokinetic/PD (PK/PD) index of effectiveness for β-lactams is the percentage of time that the free drug concentration remains greater than the MIC, fT >MIC . 3A PD index target of fT >MIC ≥ 50% is widely used to define antibacterial breakpoints for piperacillin/tazobactam, but the appropriate PD index targets for critical care patients is less clear. 4n critical care, alternative β-lactam targets have been employed, such as exceeding the β-lactam trough concentration by 4 × the pathogen MIC. 5 PD factors have also led to the adoption of prolonged or continuous infusion therapies of piperacillin/ tazobactam to treat aerobic Gram-negative rods and, most especially, Pseudomonas aeruginosa infection. 6-8However, there are burgeoning concerns regarding these 'supratherapeutic' doses, with the prevalence of β-lactam toxicity in the critical care environment thought to be underestimated. 9,10Here we aim to increase our preclinical data and evaluate the activity of piperacillin/ tazobactam against P. aeruginosa, focusing on the fT >MIC target for antibacterial effect (ABE) and emergence of resistance (EOR).
Four clinical strains of P. aeruginosa were used: 45966 (piperacillin/tazobactam MIC 4 mg/L); 46042 (MIC 6 mg/L); 27853 (MIC 4 mg/L); and 46172 (MIC 6 mg/L).MICs were obtained following EUCAST guidelines. 11Clinical strains were obtained from North Bristol NHS Trust and tested alongside the QC ATCC strain 27853 (MIC 4 mg/L).fT >MIC dose ranging from 0% to 100% fT >MIC for piperacillin/tazobactam was tested against these strains in a dilutional single compartment in vitro PK model (IVPKM).A pharmacy preparation of piperacillin/tazobactam (4 g/0.5 g; Milpharm, UK) was used for both dose ranging and piperacillin/tazobactam dose escalation simulations, with t ½ = 1 h, 8 hourly, and a minimum of eight experiments per strain performed to simulate 0%-100% fT >MIC .The fT >MIC ratios were based on the piperacillin concentrations with a ratio of 1:0.125 piperacillin:tazobactam. Mueller-Hinton broth (100%; Thermo Fisher, Basingstoke, UK) was used in all experiments.Nutrient agar plates were used to recover cfu enumerations per timepoint, 0-8 h, and every subsequent 24 h increment per IVPKM/per simulation, and blood agar plates were poured containing multiples of piperacillin/tazobactam MIC, measured in mg/L every 24 h increment from T0 to determine EOR.The inoculum was 10 6 cfu/mL and experiments were performed over 72 h.
The relationship between fT >MIC and ABE were described by log reduction in viable count at 24, 48 and 72 h.This was done using a Boltzmann sigmoid E max equation with the software package GraphPad Prism: y = bottom + (top−bottom)/ {1 + exp(V 50 −x)/slope]} (San Diego, CA, USA).EOR was assessed by changes in population profile from baseline at time 0 and 24 h by culture onto media containing ×4 and ×8 piperacillin/ tazobactam MIC.Viable counts (log cfu/mL) were determined over the 72 h of piperacillin/tazobactam exposure.
The risk of EOR, as indicated by the recovery of resistant isolates on ×4 MIC or ×8 MIC plates exhibited an inverted U pattern, demonstrating that lower doses exhibit less selective pressure.Initially the lower dose produced a small amount of resistance, and as this dose increased so did the EOR; this then continued up to a critical point whereby the bacteria were overcome by the higher concentrations of piperacillin/tazobactam.At 24 h, EOR steadily rises with the maximum risk of EOR being at an fT >MIC of 40%-60%, then decreases at higher targets, with fT >MIC of >60%-80%.The complete population profiles for ×4 MIC and ×8 MIC recovery plates are shown in Table S1 (available as Supplementary data at JAC-AMR Online).
In conclusion, an fT >MIC of 50% for piperacillin/tazobactam is associated with a −1 log reduction in bacterial counts of P. aeruginosa after 24 h exposure.fT >MIC targets of >80% are required for static or −1 log kill over 72 h.fT >MIC in the range of 40%-60% maximally amplifies resistance at 24 h, decreasing at higher targets of fT >MIC > 60%.
Our in vitro data for piperacillin/tazobactam align well with the current tenet of a PD index target of fT >MIC ≥ 50% for reduction in bacterial counts.Less studied is EOR; here, selection of resistance is most pronounced in the range of 40%-60% fT >MIC , and so prevention of EOR in vitro requires targets of fT >MIC > 60%.This data suggest that while the conventional dose of piperacillin/tazobactam can be used successfully to treat P. aeruginosa infection, dose adjustment to achieve trough concentrations of >4 × pathogen MIC may be unnecessary.In addition, it may be associated with unintended toxicity. 5To prevent EOR in vitro requires targets of fT >MIC > 60%.
This was an in vitro experiment, and the authors acknowledge the challenges in translating this to an in vivo/clinical recommendation.